Perl 7 By Default
Perl 7 has been announced as the next direction of Perl development. My previous blog post explored at a high level the risks and benefits of the announced direction, as well as those of a more incremental proposal. The primary and critical difference between these two approaches is the decision to change interpreter defaults in an incompatible manner; specifically, to have strict and warnings and possibly other features enabled by default for code that does not specify otherwise. I would like to explore each of the arguments presented for this design choice.
Optimizing For New
The primary benefit of changing the implicit defaults is, of course, to allow Perl programmers to write code in a more modern way and newcomers to program in a safer environment without having to know the sometimes arcane or niche ways to activate such an environment.
In a perfect world, a change in defaults helps with this problem. But in practice, it will be a step backwards. As it stands today, people continue to use Perl tutorials and code examples from the early 2000s, and this won't change because some new ones will have a 7 attached. The oldest examples out there will fail to run under this Perl 7 (in the best case; the worst case is more unpredictable), which is most likely to turn the newcomer away. These aren't great examples to learn Perl from in any case, but the chance of continued engagement is better than none. The vast majority of tutorials and examples will still recommend and include strict and warnings, which will behave no differently under this Perl 7.
And finally, any new material written for this Perl 7 will omit the boilerplate entirely. Since it will have no boilerplate, it will have no indication that the code was written to a Perl 7 standard, or preventing it from running on a Perl 5 interpreter, as these features do not carry any inherent incompatibility. This puts the onus on every person sharing such code to include these caveats, with no guarantee that a reader will understand such context. Even if only the bare minimum of strict and warnings are enabled by default, this code run on a Perl 5 interpreter will silently miss the compile- and run-time safety features these pragmas provide, resulting in less exposure of the modern coding environment we wish to present, not more. If other features such as
say are enabled by default in Perl 7, such code on a Perl 5 interpreter will result in the same confusing errors one currently gets when omitting the necessary feature.
Temporarily accepting the optimistic assumption that changing the programming environment based on Perl version rather than explicit declaration will be more attractive to new programmers; we should not ignore the needs of the existing userbase. Yes, Perl use is declining; no, it is not dead (as evidenced by having this discussion). There are active CPAN authors, active use of Perl in corporate codebases, 15 year old oneliners and scripts used for systems administration. New defaults do not inherently improve the experience of any of these users; they have not only existing code that may stop working, but existing expertise, code style and assumptions that affect new code they write. New defaults require re-evaluation of both of these aspects; a new version declaration requires only re-evaluating what is written or converted within that scope.
This is not a choice to optimize for new users instead of abandoned code, but a choice to optimize for appearance instead of both existing code and active users.
Why Not Both
As changing the defaults would necessarily alienate those who wish to continue using old code without having to rewrite it, it has been suggested that such users can simply continue to use Perl 5, that LTS versions of Perl 5 would be provided for many years, and further that distributors of Perl are likely to distribute separate perl5 and perl7 binaries.
If we expect Perl 7 to be provided ultimately as a separate incompatible binary, then we are expecting a fork of the Perl ecosystem. Existing code will not be guaranteed to run on the new Perl 7 binary, and new Perl 7 code will at best fail to run on the old Perl 5 binaries, and at worst silently fail to inform the user of errors. Code analysis tools like perlcritic will need to be told which Perl they are analyzing as the code will not contain any hints itself. Any discussion of Perl coding will have to start with "Are you using Perl 5 or Perl 7?"
This approach is technically feasible with an uncomfortable transition period, and would make sense if Perl 7 was to be a significant evolution of the language, and brought great benefits inherent in the incompatibility. This is simply not the case; all of the foreseeable benefits can be realized without breaking changes, and thus this design and its potential for confusion and schism is unnecessary.
The greatest risk of this approach, of course, is the scenario that the Perl 7 interpreter never reaches widespread adoption and development restarts on Perl 5. This risk should not be taken lightly, particularly as this approach seems predicated on the expectation that most people will in fact stick with Perl 5 if Perl 7 is not compatible. We should certainly learn from the Raku saga, and not follow the same path without even a new language to offer this time.
A New Culture
Since the new defaults will not contain any revolutionary new feature or ability, a Perl 7 with changed defaults would not provide sufficient value inherent in the incompatibility. This has been rationalized with a secondary goal to change the culture of Perl development. The current Perl policy is one that it has held for decades: backwards compatibility is paramount, despite the inconvenience. It is suggested that future Perl releases will in fact follow the Perl 7 model to a greater extent, and thus the Perl 7 changes to defaults are designed to set a precedent of major versions breaking compatibility, possibly without the restrained nature of the currently suggested changes.
This seems short-sighted. By all measures, all of the features that could be introduced in future major versions will similarly be easy to introduce in a non-breaking manner. And more importantly, do we really want to go down the road of releasing major versions that break code as a policy rather than when it is necessary and beneficial? This approach does not seem to be in line with Perl's defining pragmatism. Perl is a known quantity, for better or worse; we cannot reinvent what Perl means to the world, only how we present its reality.
They Did It
Python and other languages have experienced major versions with significant breaking changes. After a long and arduous transition period, the Python community is just now beginning to coalesce around Python 3. This appears to show that major versions can break compatibility and the end result can justify the difficult transition.
But the comparison is flawed. The Python 2 to 3 change primarily heralded a one-time incompatible change of explicitly marking strings as bytes or characters, resulting in an objectively better language feature that can only work when applied throughout the whole stack of code. Perl has no feature that even approaches implementing an explicit distinction like Python added, and all of Perl's existing features are easily able to be limited to a lexical scope and leave other code unmodified. Thus, Perl has no feature that would benefit from the significant risk of this approach in the way that Python did. And far from the idea that it facilitates a culture of breaking changes in major versions, the ordeal of Python 3 has very likely made developers more apprehensive of future breaking versions so as not to repeat the trauma without good reason.
Another suggested impetus for incompatible major versions is to allow Perl development to continue more smoothly, without the shackles of past cruft.
It is of course a worthwhile goal, but changing interpreter defaults does not assist in this goal. Changing defaults does not affect policy or practical ability to remove or alter actual language features. Of course the established policy and application of it can be debated, and the ability to leverage major versions provides significant opportunity in this regard, but this discussion is unrelated to the choice to change user-facing defaults.
Further, it has been pointed out by two of the three people that have recently been working on forward-moving language features that user-facing features and syntax are not a significant barrier to such development. See these messages from Dave Mitchell, who has been working on the signatures feature, and Paul Evans, who recently implemented the
isa feature and has many more planned including core try/catch. (Zefram, who recently implemented chained comparisons, has not weighed in on the discussion to my knowledge.)
Moving forward in Perl development and culture would be best achieved by the simple and consistent approach of leveraging the
use v7 declaration to provide a modern environment lexically, and interpreting code without such a declaration the same way as now in Perl 5.
The major version provides significant opportunity for messaging of new features, as well as what features such a declaration will enable. The announcement of Perl 7 can detail all of the great things you get in a scope with the line
use v7. It is significantly easier to remember that, hypothetically,
use v7 will enable warnings and
use v8 will enable signatures, than to remember that
use v5.12 enables strict and
use v5.16 enables
__SUB__. The declaration provides explicit reference that the code within that file or code snippet requires that version of Perl, without relying on context of shebangs, filenames, or surrounding text that can get left out. And it provides an explicit hint to static parsers (syntax highlighters, perlcritic, etc) how the following code should be interpreted. Someday, when we are ready for a Perl 8, it will not require repeating any of the plans discussed above for introducing incremental incompatibility; it will only require
I believe making good use of a new major version is extremely important to portraying the continued and forward development of Perl to the wider programming community. A major version with major features can be a significant boon to jumpstart the stagnating perception of Perl and bring it in line with the reality of its development. But changing the interpreter defaults is unnecessary, irresponsible, and counterproductive. A simple scoped and versioned declaration to enable the modern coding environment provides a stronger way forward and no new risk. It allows Perl 5 users and Perl 7 users to just be Perl users. It should be our default choice.